microRNA function in epidermal and hair follicle morphogenesis and homeostasis is an exciting new area of skin biology that has major implications for our understanding of skin and hair follicle function and skin disorders including alopecias, skin tumorigenesis, and wound healing defects. The experiments proposed here use in vivo and in vitro techniques to more precisely define the global roles of miRNAs in skin homeostasis, and to begin to identify specific miRNAs and their target genes that are critical for normal skin function. As such we believe that our studies will have major significance for our understanding of skin biology and have the potential to ultimately reveal therapeutic targets for a range of diseases from alopecias to skin tumors. The recent discovery of post-transcriptional regulation by microRNAs (miRNAs) revealed a previously unsuspected mechanism by which gene expression is controlled and refined. Non-protein coding miRNA precursors (pri-miRNAs) are transcribed from nuclear genes and form distinctive stem-loop structures that are specifically recognized and cleaved by the nuclear enzyme Drosha and its cofactor DGCR8 to form pre-miRNAs. Pre-miRNAs are exported from the nucleus and processed to approximately 22 nucleotide mature miRNA by the Dicer enzyme. miRNAs are assembled into the RISC complex which regulates gene expression by inhibiting the translation of, and/or destabilizing, target mRNAs that contain partially complementary sequences in their 3'non-coding regions. Several hundred miRNAs have been identified in mammals, and at least 33 are highly expressed in the skin. Constitutive epidermal deletion of the unique mouse Dicer gene results in loss of mature miRNAs and multiple phenotypes including: failure of morphogenesis and maintenance of hair follicles;absence of expression of hair follicle stem cell markers;disrupted epidermal-dermal signaling;evagination of hair follicle dermal papillae;and epidermal hyperproliferation and apoptosis. Based on these phenotypes we hypothesize that Dicer is required postnatally for maintaining hair follicle growth and epidermal homeostasis, and that it functions, in part, within hair follicle stem cells. We further hypothesize that epidermal Dicer mutant phenotypes result from miRNA depletion. To test these hypotheses, we will: (i) inducibly delete epidermal Dicer in postnatal skin;(ii) delete Dicer specifically within hair follicle stem cells;(iii) determine whether epidermal depletion of another essential miRNA processing pathway component replicates the effects of loss of epidermal Dicer;(iv) characterize miRNAs expressed in different skin compartments and begin to identify their target mRNAs.